1.Field of the Invention
The present invention relates to an image forming apparatus, and more specifically, to an image forming apparatus having a conveyance mechanism for conveying a sheet onto which a toner image has been transferred, while forming a loop between a transfer unit and a fixing unit.
2.Description of the Related Art
Conventionally, in an image forming apparatus employing an electrophotographic process, a toner image formed on an image bearing member is transferred onto a sheet in a transfer unit, and subsequently the sheet is heated in a fixing unit equipped with a fixing roller pair to fix the toner image on the sheet. Further, there is an image forming apparatus which forms images on both sides of the sheet. In this type of an image forming apparatus, a sheet with a toner image formed on its first surface is reversed, and another toner image is transferred and fixed on a second surface of the sheet. Thus, the images are formed on both sides of the sheet.
In such a conventional image forming apparatus, it is possible that when the leading edge of the sheet is entering into a fixing unit, a trailing edge has not yet passed through a transfer unit depending on the length of a sheet. Generally, a sheet conveyance speed of the fixing unit and a sheet conveyance speed of the transfer unit are set to be substantially equal. However, there may appear a difference between sheet conveyance speeds of the fixing unit and the transfer unit due to thermal expansion, individual difference of apparatus parts, or aging of a fixing roller provided in the fixing unit.
When a sheet conveyance speed of the fixing unit is higher than a sheet conveyance speed of the transfer unit, there can occur a phenomenon that a sheet that is bearing unfixed toner image is pulled toward the fixing unit side between the fixing unit and the transfer unit. Such a phenomenon may invite defective transfer and image deterioration. Thus, in order to prevent such a phenomenon that the sheet comes under a tensile stress, a difference of the sheet conveyance speeds is provided between the fixing unit and the transfer unit, so as to warp the sheet that is to be conveyed between the transfer unit and the fixing unit to form a loop.
In the image forming apparatus, heaters are arranged inside heating rollers that constitute a fixing roller pair, and temperature control of the heater is performed during the fixing process of a toner image. Owing to the temperature control, a surface of the heating roller is held at a predetermined fixing temperature. It is known that quality of an image on a sheet varies depending on heat quantity and pressure energy received from the fixing roller pair. Therefore, image quality and gloss are more likely to be affected in a color image because a large amount of toner adheres to the sheet due to heat quantity and pressure energy received from the fixing roller pair, compared with a black and white image to which a small amount of toner adheres.
For example, when an image forming operation is executed in succession, the fixing process of the next sheet may be started while the energy lost by a fixing process of a prior sheet has not yet been sufficiently recovered and the temperature of the heating roller is still low. Such a variation of heat quantity may have an influence on image quality on a sheet. For example, image quality may change among images on one piece of sheet, or image quality of each sheet may be different with each other even when the similar images are formed in succession. To address such a problem, electric power supplied to the heater is increased (i.e., heater capacity is increased) so that supplied energy and consumed energy become balanced as viewed over a relatively longer period of time. However, if materials with low thermal responsiveness such as heat-resistant rubber are used on the surface of the heating roller to enhance image quality, the temperature of the heating roller may drop.
Thus, when temperature on the heating roller changes, the change in temperature causes a change in outer diameter of the heating roller, so that a conveyance speed of the sheet passing through a fixing roller pair also is changed. In addition, a conveyance speed of the sheet varies depending on difference in heat capacity of the sheet, in other words, depending on whether the sheet is thin or thick, or overhead projector (OHP) sheet. Thus, if a sheet conveyance speed of the fixing roller pair varies, it becomes difficult to form and maintain a stable loop between the transfer unit and the fixing unit, particularly in an apparatus in which the length of a sheet conveyance path for guiding the sheet between the transfer unit and the fixing unit is set short.
Further, if a sheet conveyance speed in the fixing unit (the fixing roller pair) is slower than a sheet conveyance speed in the transfer unit, an excessive loop will be formed. If such an excessive loop is formed, the sheet comes into contact with a conveyance guide that constitutes the sheet conveyance path, thus causing poor images and transfer noises (irregularities of images caused at the time of transfer). In order to solve such a problem, Japanese Patent Application Laid-Open No. 07-234604 discusses a technique to detect a size of a loop of a sheet formed between a transfer unit and a fixing unit, and to increase/decrease a sheet conveyance speed of a fixing roller pair depending on a size of the loop.
FIG. 8 is a view illustrating a configuration of a transfer unit and a fixing unit of the conventional image forming apparatus. The configuration includes a transfer unit 113 and a fixing unit 105 equipped with a fixing roller pair having a heating roller 105a and a pressure roller 105b. In a sheet conveyance path 120 between the transfer unit 113 and the fixing unit 105, a detecting lever 150 is arranged. The detecting lever 150 is arranged such that when a loop is formed on a sheet P, the detecting lever rotates around a rotating shaft 151 being pressed by the sheet P, and a sensor S is turned on/off by the rotation of the detecting lever 150.
Thus, the sensor S detects the rotation of the detecting lever 150 and determines that the loop of the sheet P has reached a predetermined size. According to the determination, a sheet conveyance speed attained by the fixing roller pair (105a and 105b) of the fixing unit 105 is controlled. For example, when the sensor S determines that the loop has reached a predetermined size, the size of the loop is decreased by increasing a sheet conveyance speed of the fixing roller pair (105a and 105b). In contrast, if the sensor S ceases to detect the rotation of the detecting lever 150, the sensor S determines that the size of loop has become too small, and then the size of loop is increased by reducing a sheet conveyance speed of the fixing roller pair (105a and 105b). Thereby, the size of the loop of the sheet P can be maintained within a predetermined range and can be stabilized.
In such a conventional image forming apparatus, a toner image on a sheet is unfixed between the transfer unit and the fixing unit, and it is necessary to convey the sheet while forming the loop in the sheet that is being conveyed. Accordingly, a spacing between a guide 122 at an image surface side and a guide 121 at a non-image surface side that constitute the sheet conveyance path 120 is set wide. However, if a spacing between the guides 121 and 122 is thus made wide, a conveyance track of the sheet P will be disturbed when the sheet P abuts against the detecting lever 150 while the sheet P is pinched and conveyed only by one of the fixing unit 105 and the transfer unit 113.
For example, when the sheet P is conveyed while a leading edge of a sheet has not yet reached the fixing unit 105, the sheet P is pinched and conveyed only by the transfer unit 113, which brings about an unstable state. If the detecting lever 150 abuts against the sheet P in that state, the sheet P may be distorted. In particular, if the sheet P has a low rigidity, this phenomenon occurs remarkably. Then, when a sheet's leading edge enters into a nip portion of the fixing roller pair 105a and 105 while the sheet P is distorted, timing of sheet entry into the nip portion of the fixing roller pair 105a and 105b, may be shifted in a width direction orthogonal to a sheet conveying direction. If the timing of the entry is thus shifted, the sheet P is skewed, which causes distortion of a toner image which is to be transferred in the transfer unit 113.
On the other hand, after a sheet's trailing edge has come out of the transfer unit 113, the sheet P is pinched and conveyed only by the fixing roller pair 105a and 105b, which brings about an unstable state. Under this state, if the detecting lever 150 is situated in a retracting position from the sheet conveyance path 120 under pressure of the warped sheet P, the sheet's trailing edge may be pressed by a backwards reaction force of the detecting lever 150 toward the guide 122 at an image surface side and abuts against the guide 122. If the sheet's trailing edge portion abuts against the guide 122 at the image surface side in this manner, yet-unfixed toner image flies off, an attitude of the sheet at the fixing unit 105 changes, and thus uneven fixation of images can occur in that portion. As a result, image quality deteriorates, and a stable image cannot be formed.